This invention relates to a novel synthesis of certain key prostaglandin precursors. In particular, it relates to the synthesis of lactone ester alcohol (8). Several routes already exist for the preparation of this alcohol first described by E. J. Corey in the Journal of American Chemical Society Vol. 93, p 1491 (1971). However all of the existing methods for the preparation of this important intermediate involve many steps which frequently give rise to reduced yields. The sequences of the present invention are short and proceed in high yield and require an inexpensive and readily available starting material.
The prostaglandins are C-20 unsaturated fatty acids which exhibit diverse physiological effects. For instance, the prostaglandins of the E and A series are potent vasodilators (Bergstrom, et al., Acta Physiol. Scand. 64:332-33 1965 and Bergstrom, et al., Life Sci. 6:449-455, 1967) and lower systemic arterial blood pressure (vasodepression) on intravenous administration (Weeks and King, Federation Proc. 23:327, 1964; Bergstrom, et al., 1965, op. cit.; Carlson, et al., Acta Med. Scand. 183:423-430, 1968; and Carlson, et al., Acta Physiol. Scand. 75:161-169, 1969). Another well known physiological action for PGE.sub.1 and PGE.sub.2 is as a bronchodilator (Cuthbert, Brit. Med. J. 4:723-726, 1969).
Still another important physiological role for the natural prostaglandins is in connection with the reproductive cycle. PGE.sub.2 is known to possess the ability to induce labor (Karim, et al., J. Obstet. Gynaec. Brit. Cwlth. 77:200-210, 1970), to induce therapeutic abortion (Bygdeman, et al., Contraception, 4, 293 (1971) and to be useful for control of fertility (Karim, Contraception, 3, 173 (1971)). Patents have been obtained for several prostaglandins of the E and F series as inducers of labor in mammals (Belgian Patent No. 754,158 and West German Patent No. 2,034,641), and on PGF.sub.1, F.sub.2, and F.sub.3 for control of the reproductive cycle (South African Patent 69/6089). It has been shown that luteolysis can take place as a result of administration of PGF.sub.2.alpha. [Labhsetwar, Nature 230 528 (1971)] and hence prostaglandins have utility for fertility control by a process in which smooth muscle stimulation is not necessary.
Still other known physiological activities for PGE.sub.1 are in the inhibition of gastric acid secretion (Shaw and Ramwell, In: Worcester Symp. on Prostaglandins, New York, Wiley, 1968, p. 55 -64) and also on platelet aggregation (Emmons, et al., Brit. Med. J. 2:468-472, 1967).
It is now known that such physiological effects will be produced in vivo for only a short period, following the administration of a prostaglandin. A substantial body of evidence indicates that the reason for this rapid cessation of activity is that the natural prostaglandins are quickly and efficiently metabolically deactivated by .beta.-oxidation of the carboxylic acid side-chain and by oxidation of the 15.alpha.-hydroxyl group (Anggard, et al., Acta. Physiol. Scand. 81, 396 (1971) and references cited therein). It has been shown that placing a 15-alkyl group in the prostaglandins has the effect of increasing the duration of action possibly by preventing the oxidation of the C15-hydroxyl [Yankee and Bundy, JACS 94, 3651 (1972)], Kirton and Forbes, Prostaglandins, 1, 319 (1972).
It was, of course, considered desirable to devise new synthetic sequences which would be shorter and more efficient than the previously existing methods. In particular, sequences which required simple starting materials and did not require complex isolation procedures or long and tedious purifications for the various intermediates as do the existing prostaglandin synthesis were sought.